1. Field of the Invention
The present invention relates to turbine engines, and more particularly to ceramic stator vane assemblies.
2. Description of the Prior Art
The problem is well known. Ceramic stator vanes would make excellent stator vanes in the turbine section of the engine. High strength ceramics have a much higher melting temperature than most known metal alloys. Metal alloy stator vanes require sophisticated cooling systems which usually means providing intricate cooling passages in the airfoils of the stator vanes. These cooling systems are often impractical when dealing with small scale turbine engines having vane airfoils with a height of approximately 1 inch. Further, if cooling systems are provided, the vanes must be enlarged in order to provide the cooling passages, thus compromising the aerodynamic performance of the airfoil. Finally, such vanes are very expensive to fabricate.
Ceramic stator vanes, on the other hand, do not require the cooling passages of an alloy vane and thus can be made lighter and more aerodynamically efficient. However, these known ceramics cannot be subjected to very high tensile stresses. On the other hand, ceramic material can be subjected to high compressive stresses before deteriorating. Attempts have been made, therefore, to mount ceramic vanes under compression. Such attempts are illustrated, for instance, in U.S. Pat. No. 4,076,451, issued Feb. 28, 1978 to Alan L. Jankot. In this patent, the compressive forces on the ceramic vane assembly are provided by a continuous metal shroud or ring and the inherent expansion of the metal ring. In an environment contemplated, the temperature in the gas path would be well in the 2500.degree. F. average. Such temperatures would be rapidly transmitted to the extremities of the vanes, and thus the metal rings surrounding the vanes would melt unless they were subjected to a cooling flow, which would again defeat the initial purpose of using the ceramic vanes. Cooling systems, of course, use air within the engine which has been compressed, and thus, if such compressed air is used for cooling, it has the same effect as leakage, which is an energy loss, thus reducing the efficiency of the engine. If the thermal conductivity of the ceramic material is high, the cooling of the peripheral extremities thereof would create serious thermal stresses within the ceramic vanes.
U.S. Pat. 3,966,353, issued June 29, 1976 to Claude R. Booher, Jr. et al, describes a ceramic vane ring assembly utilizing a multi-component system with insulating pads and spring devices or other for maintaining the vanes under compression. As evident from the Booher, Jr. et al patent, much leakage would occur surrounding the vane assembly. The same can be said for U.S. Pat. 3,857,649, issued Dec. 31, 1974 to Richard J. Schaller et al.